How does attention affect perception? The long-term objective of the research is to contribute to the understanding of visual attention by studying the effects of covert attention on early vision, drawing on theoretical work in perception, cognition, and neurophysiology, and using methods from visual psychophysics. There are two systems of covert attention: 'transient' (exogenous or stimulus-driven) and 'sustained' (endogenous or conceptually-driven). Recent research has shown that transient attention affects early vision by enhancing contrast sensitivity, increasing spatial resolution, and accelerating information accrual. To date, studies on sustained attention have focused on higher cognitive processes, whereas relatively little is known about its effects on early vision. The proposed research will systematically investigate the effects of both attentional systems on early vision. Research has shown that transient attention, being automatic, may result in impaired performance or deviate us from veridical perception. A main objective of the current proposal is to test the hypothesis that sustained attention is more flexible and hence may adapt to task demands better than transient attention. The specific aims are to investigate: (1) the effects of transient and sustained attention on contrast sensitivity by evaluating the quality of the signal at both the attended and unattended locations across the visual field, examining the time course of attentional effects on contrast sensitivity, and exploring how the gain mechanisms of attention and selective adaptation affect stimulus processing; (2) the effects of transient and sustained attention on spatial resolution at the attended and unattended locations and whether attention affects the channels' spatial frequency tuning by increasing sensitivity to higher frequencies; (3) whether transient and sustained attention differentially affect the gain and tuning of 1st and 2nd order spatial frequency channels; (4) the levels at which transient and sustained attention exert their effects on contrast sensitivity and spatial resolution by studying binocular rivalry; (5) the effects of covert attention on apparent contrast and spatial resolution during standard binocular viewing and binocular rivalry. Results will be interpreted within the theoretical framework of an ensemble of spatial filters operating in linear and nonlinear regimes. The findings have implications for human factors, as well as for our understanding of many neuropsychological conditions, e.g. unilateral neglect, schizophrenia, and ADHD. [unreadable] [unreadable] [unreadable]